It's a marketing tactic called "shadow-dropping". It's a risky tactic that relies on word-of-mouth for marketing instead of an expensive ad campaign. It tends to be used either when the company isn't confident in their product, or if the drop is so desirable that it'll sell well regardless of marketing.

We can assume this is the latter, though it's possible Bethesda were also hedging their bets due to the remake's lack of mod compatibility since long-tail revenue from modding basically carries them for the several years between game releases.